Method and system for improved dilution purging

ABSTRACT

Methods and systems are provided for expediting EGR purging in a hybrid vehicle during transient operations, such as tip-out to lower load conditions. In response to decreasing engine torque demand, engine fueling is disabled and a motor is used to spin the engine unfueled until a desired LP-EGR rate is achieved. Alternatively, engine operation is maintained with EGR disabled until the desired LP-EGR rate is achieved, and the excess engine torque generated is stored in a system battery.

TECHNICAL FIELD

The present application relates to methods and systems for improvingpurging of low pressure EGR from an engine during operation at lightloads.

BACKGROUND AND SUMMARY

Exhaust gas recirculation (EGR) systems recirculate a portion of exhaustgas from an engine exhaust to an engine intake system to improve fueleconomy and vehicle emissions by reducing throttling losses andcombustion temperatures. In turbo-charged direct injection engines, alow-pressure EGR (LP-EGR) circuit may be implemented. The LP-EGR circuitrecirculates exhaust gases from an exhaust passage downstream of aturbine to an intake passage upstream of a turbocharger compressor.

However, due to the pre-compressor location of EGR delivery, there may asignificant transport delay between the EGR valve and the combustionchamber. Specifically, the exhaust residuals may need to travel thoughthe turbocharger compressor, high-pressure air induction plumbing,charge air cooler, and intake manifold before reaching the combustionchamber. As a result of the transport delay, during conditions when EGRneeds to be rapidly reduced, such as during a tip-out to low loadconditions, there may be more dilution in the intake than desired. Thepresence of increased intake-air dilution at low loads can increasecombustion stability issues and the propensity for engine misfires.

One example approach for addressing the extra residuals is shown by Maet al. in U.S. Pat. No. 6,014,959. Therein, a rigid connection isprovided between an EGR throttle and a main air intake throttle, linkingmovement of the EGR throttle as a function of the movement of the mainthrottle. This allows EGR dilution to be always provided in a fixedproportion to the intake airflow.

However, the inventors herein have recognized potential issues with suchan approach. As an example, the transport delay may not be sufficientlyaddressed while the fuel economy benefits of LP-EGR are limited. Forexample, the linking of EGR dilution to intake airflow may result inLP-EGR being provided at some low load points where no fuel economybenefit from the EGR is achieved. In some cases, there may even be afuel penalty associated with the delivery of LP-EGR at the low loadpoint. As such, it may not be possible to rapidly purge the LP-EGR fromthe intake in such systems without affecting airflow. As anotherexample, the lower load points may limit the delivery of EGR at higherload points as they are the points where the combustion system is mostdilution limited. As such, this can limit the peak EGR rates achievableduring high loads. The presence of excess dilution in the engine intakesystem can also render the compressor susceptible to corrosion andcondensation from the lingering EGR. Furthermore, increased condensationmay occur at a charge air cooler of a boosted engine system due to theflow of EGR through the cooler. The increased condensation maynecessitate additional counter-condensation measures.

The inventors have recognized that at least some of the above issues maybe addressed by operating a hybrid vehicle system in a battery chargingmode to rapidly purge LP-EGR. In one example, this is achieved by amethod for a hybrid vehicle system comprising: in response to decreasingengine torque demand while operating an engine with EGR, disabling EGR,and until EGR in an engine intake is lower than a threshold, maintainingengine operation with EGR disabled and charging a system battery withthe excess engine torque generated. In this way, LP-EGR can be rapidlypurged without affecting torque to the wheels.

As an example, during medium to high load conditions, a hybrid vehiclesystem may be operated in an engine mode with the engine combusting toprovide engine torque for propelling the vehicle wheels. Further, duringthe engine mode, low pressure EGR (LP-EGR) may be flowing from theengine exhaust to the engine intake to provide addition fuel economy andemissions benefits. In response to a tip-out to lower load conditions,EGR may be disabled by closing an EGR valve in an LP-EGR passage. Byoperating the engine with the EGR valve closed, EGR in the intake can berapidly replaced with fresh intake air. As such, during the engineoperation, the engine torque generated may be more than the demandedengine torque. The excess engine torque generated may be stored in asystem battery if the battery has sufficient charge acceptingcapability. For example, the excess engine torque may be used to drive amotor/generator coupled to the battery. Engine operation and batterycharging may be continued until the LP-EGR level is below a thresholdlevel (e.g., all the LP-EGR has been replaced with fresh intake air).Thereafter, the engine may be shut down and the vehicle may be propelledvia motor torque.

In this way, EGR purging from an engine intake can be expedited. Byoperating the engine with an EGR valve closed, intake EGR can bereplaced with intake air. In addition, the higher engine torque can beadvantageously used to charge a system battery. As such, this allowshigher engine torque and higher battery charge to be held while EGR ispurged. By rapidly reducing the intake EGR level at low load conditions,higher EGR rates can be achieved when the engine is subsequentlyrestarted. As such, this substantially improves engine efficiency,particularly in medium to high engine speed-load regions. By replacingthe EGR with fresh air, evaporation of water and hydrocarbon condensatesis increased, reducing their concentration in the engine, and the needfor counter-condensation measures. In addition, the reduction incondensation reduces compressor and charge air cooler corrosion anddegradation. Overall, boosted engine performance is improved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of a hybrid electric vehicle system.

FIG. 2 shows a schematic depiction of a combustion chamber of an enginesystem of FIG. 1.

FIG. 3 depicts an example engine system configured with exhaust gasrecirculation (EGR) capabilities.

FIG. 4 shows a high level flow chart for operating the hybrid vehiclesystem of FIG. 1 to expedite purging of low pressure EGR.

FIG. 5 shows example EGR schedules at selected engine speed-loadconditions.

FIG. 6 shows example operations in a hybrid electric vehicle to expediteEGR purging, according to the present disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for using amotor/generator of a hybrid vehicle system (such as the vehicle systemof FIG. 1) to expedite purging of EGR from an engine (such as the enginesystem of FIGS. 2-3) at low load conditions. While operating the hybridvehicle in an engine mode, with EGR flowing, in response to decreasingengine load, EGR delivery to the engine may need to be rapidly reduced.During such conditions, a controller may be configured to perform acontrol routine, such as the example routine of FIG. 4, to disable fuelto the engine and propel the vehicle using motor torque. In addition,the controller may spin the engine, unfueled, using motor torque, for aduration until EGR is sufficiently purged from the engine's intakemanifold. Alternatively, if the vehicle system battery is capable ofaccepting charge, the controller may disable EGR and operate the enginewith the EGR valve closed while storing the excess engine torquegenerated as battery charge. Example adjustments are shown withreference to FIG. 6. In this way, during subsequent engine operation,higher EGR schedules may be achieved, particularly at medium to highengine speed-load conditions (FIG. 5). Overall, engine performance isimproved.

FIG. 1 depicts a hybrid propulsion system 100 for a vehicle. In thedepicted embodiment, the vehicle is a hybrid electric vehicle (HEV).Propulsion system 100 includes an internal combustion engine 10 having aplurality of cylinders 30. Fuel may be provided to each cylinder ofengine 10 from a fuel system (not shown) including one or more fueltanks, one or more fuel pumps, and injectors 66.

Engine 10 delivers power to transmission 44 via torque input shaft 18.In the depicted example, transmission 44 is a power-split transmission(or transaxle) that includes a planetary gearset 22 and one or morerotating gear elements. Transmission 44 further includes an electricgenerator 24 and an electric motor 26. The electric generator 24 and theelectric motor 26 may also be referred to as electric machines as eachmay operate as either a motor or a generator. Torque is output fromtransmission 44, for propelling vehicle tractions wheels 52, via a powertransfer gearing 34, a torque output shaft 19, and differential-and-axleassembly 36.

Generator 24 is drivably connected to electric motor 26 such that eachof electric generator 24 and electric motor 26 may be operated usingelectric energy from an electrical energy storage device, hereindepicted as battery 54. In some embodiments, an energy conversiondevice, such as an inverter, may be coupled between the battery and themotor to convert the DC output of the battery into an AC output for useby motor. However, in alternate embodiments, the inverter may beconfigured in the electric motor.

Electric motor 26 may be operated in a regenerative mode, that is, as agenerator, to absorb energy from vehicle motion and/or the engine andconvert the absorbed kinetic energy to an energy form suitable forstorage in battery 54. Furthermore, electric motor 26 may be operated asa motor or generator, as required, to augment or absorb torque providedby the engine.

Planetary gearset 22 comprises a ring gear 42, a sun gear 43, and aplanetary carrier assembly 46. The ring gear and sun gear may be coupledto each other via the carrier. A first input side of planetary gearset22 is coupled to engine 10 while a second input side of the planetarygearset 22 is coupled to the generator 24. An output side of theplanetary gearset is coupled to vehicle traction wheels 52 via powertransfer gearing 34 including one or more meshing gear elements 60-68.In one example, the meshing gear elements 60-68 may be step ratio gearswherein carrier assembly 46 may distribute torque to the step ratiogears. Gear elements 62, 64, and 66 are mounted on a countershaft 17with gear element 64 engaging an electric motor-driven gear element 70.Electric motor 26 drives gear element 70, which acts as a torque inputfor the countershaft gearing. In this way, the planetary carrier 46 (andconsequently the engine and generator) may be coupled to the vehiclewheels and the motor via one or more gear elements. Hybrid propulsionsystem 100 may be operated in various embodiments including a fullhybrid system, wherein the vehicle is driven by only the engine andgenerator cooperatively, or only the electric motor, or a combination.Alternatively, assist or mild hybrid embodiments may also be employed,wherein the engine is the primary source of torque and the electricmotor selectively adds torque during specific conditions, such as duringa tip-in event.

For example, the vehicle may be driven in an engine mode wherein engine10 is operated in conjunction with the electric generator (whichprovides reaction torque to the planetary gearset and allows a netplanetary output torque for propulsion) and used as the primary sourceof torque for powering wheels 52 (the generator may also be providingtorque to wheels if in motoring mode). During the engine mode, fuel maybe supplied to engine 10 from a fuel tank via fuel injector 66 so thatthe engine can spin fueled to provide the torque for propelling thevehicle. Specifically, engine power is delivered to the ring gear of theplanetary gearset. Coincidentally, the generator provides torque to thesun gear 43, producing a reaction torque to the engine. Consequently,torque is output by the planetary carrier to gears 62, 64, 66 oncountershaft 17, which in turn delivers the power to wheels 52.Additionally, the engine can be operated to output more torque than isneeded for propulsion, in which case the additional power is absorbed bythe generator (in generating mode) to charge the battery 54 or supplyelectrical power for other vehicle loads.

In another example, the vehicle may be driven in an assist mode whereinengine 10 is operated and used as the primary source of torque forpowering wheels 52 and the electric motor is used as an additionaltorque source to act in cooperation with, and supplement the torqueprovided by, engine 10. During the assist mode, as in the engine mode,fuel is supplied to engine 10 so as to spin the engine fueled andprovide torque to the vehicle wheels.

In still another example, the vehicle may be driven in an engine-off orelectric mode wherein battery-powered electric motor 26 is operated andused as the only source of torque for driving wheels 52. As such, duringthe electric mode, no fuel may be injected into engine 10 irrespectiveof whether the engine is spinning or not. The electric mode may beemployed, for example, during braking, low speeds, low loads, whilestopped at traffic lights, etc. Specifically, motor power is deliveredto gear element 70, which in turn drives the gear elements oncountershaft 17, and thereon drives wheels 52.

Propulsion system 100 may further include a control system includingcontroller 12 configured to receive information from a plurality ofsensors 16 (various examples of which are described herein) and sendingcontrol signals to a plurality of actuators 81 (various examples ofwhich are described herein). As one example, sensors 16 may includevarious pressure and temperature sensors, a fuel level sensor, variousexhaust gas sensors, etc. The various actuators may include, forexample, the gear set, cylinder fuel injectors (not shown), an airintake throttle coupled to the engine intake manifold (not shown), etc.Additional sensors and actuators are elaborated at FIGS. 2-3. Controller12 may receive input data from the various sensors, process the inputdata, and trigger the actuators in response to the processed input databased on instruction or code programmed therein corresponding to one ormore routines. An example control routine is described herein withregard to FIG. 4.

FIG. 2 depicts an example embodiment of a combustion chamber or cylinderof engine 10 (of FIG. 1). Engine 10 may receive control parameters froma control system including controller 12 and input from a vehicleoperator 130 via an input device 132. In this example, input device 132includes an accelerator pedal and a pedal position sensor 134 forgenerating a proportional pedal position signal PP. Cylinder (hereinalso “combustion chamber”) 30 of engine 10 may include combustionchamber walls 136 with piston 138 positioned therein. Piston 138 may becoupled to crankshaft 140 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. Crankshaft 140 maybe coupled to at least one drive wheel of the passenger vehicle via atransmission system. Further, a starter motor may be coupled tocrankshaft 140 via a flywheel to enable a starting operation of engine10. Specifically, the generator 24 (of FIG. 1) and driveline includingmotor 26 (of FIG. 1) may be coupled to the crankshaft and provide torquefor engine cranking.

Cylinder 30 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 30. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 2 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 20 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 20 may be disposed downstream ofcompressor 174 as shown in FIG. 2, or alternatively may be providedupstream of compressor 174. In some embodiments, as elaborated withreference to FIG. 3, a charge air cooler (CAC) may be located downstreamof compressor 174 and upstream of throttle 20 for cooling a boostedaircharge delivered to the engine. Alternatively, the CAC can be locateddownstream of the throttle integrated in the intake manifold 146.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 30. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

Exhaust temperature may be estimated by one or more temperature sensors(not shown) located in exhaust passage 148. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 128. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 30 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 30. In some embodiments, eachcylinder of engine 10, including cylinder 30, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 by cam actuation viacam actuation system 151. Similarly, exhaust valve 156 may be controlledby controller 12 via cam actuation system 153. Cam actuation systems 151and 153 may each include one or more cams and may utilize one or more ofcam profile switching (CPS), variable cam timing (VCT), variable valvetiming (VVT) and/or variable valve lift (VVL) systems that may beoperated by controller 12 to vary valve operation. The position ofintake valve 150 and exhaust valve 156 may be determined by valveposition sensors 155 and 157, respectively. In alternative embodiments,the intake and/or exhaust valve may be controlled by electric valveactuation. For example, cylinder 30 may alternatively include an intakevalve controlled via electric valve actuation and an exhaust valvecontrolled via cam actuation including CPS and/or VCT systems. In stillother embodiments, the intake and exhaust valves may be controlled by acommon valve actuator or actuation system, or a variable valve timingactuator or actuation system.

Cylinder 30 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 13:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 30 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more injectors for providing a knock or pre-ignition suppressingfluid thereto. In some embodiments, the fluid may be a fuel, wherein theinjector is also referred to as a fuel injector. As a non-limitingexample, cylinder 30 is shown including one fuel injector 166. Fuelinjector 166 is shown coupled directly to cylinder 30 for injecting fueldirectly therein in proportion to the pulse width of signal FPW receivedfrom controller 12 via electronic driver 168. In this manner, fuelinjector 166 provides what is known as direct injection (hereafter alsoreferred to as “DI”) of fuel into combustion cylinder 30. While FIG. 2shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may improve mixing and combustion when operating the enginewith an alcohol-based fuel due to the lower volatility of somealcohol-based fuels. Alternatively, the injector may be located overheadand near the intake valve to improve mixing.

Fuel may be delivered to fuel injector 166 from a high pressure fuelsystem 8 including fuel tanks, fuel pumps, and a fuel rail.Alternatively, fuel may be delivered by a single stage fuel pump atlower pressure, in which case the timing of the direct fuel injectionmay be more limited during the compression stroke than if a highpressure fuel system is used. Further, while not shown, the fuel tanksmay have a pressure transducer providing a signal to controller 12. Itwill be appreciated that, in an alternate embodiment, injector 166 maybe a port injector providing fuel into the intake port upstream ofcylinder 30.

As described above, FIG. 2 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel tanks in fuel system 8 may hold fuel with different qualities, suchas different compositions. These differences may include differentalcohol content, different octane, different heat of vaporizations,different fuel blends, and/or combinations thereof etc. In one example,fuels with different alcohol contents could include one fuel beinggasoline and the other being ethanol or methanol. In another example,the engine may use gasoline as a first substance and an alcoholcontaining fuel blend such as E85 (which is approximately 85% ethanoland 15% gasoline) or M85 (which is approximately 85% methanol and 15%gasoline) as a second substance. Other alcohol containing fuels could bea mixture of alcohol and water, a mixture of alcohol, water and gasolineetc.

Further, in the disclosed embodiments, an EGR system may route a desiredportion of exhaust gas from exhaust passage 148 to air induction passage142. FIG. 2 shows an LP-EGR system wherein LP-EGR is routed throughLP-EGR passage 240 from downstream of turbine 176 to upstream ofcompressor 174. The amount of LP-EGR provided to intake passage 142 maybe varied by controller 12 via LP-EGR valve 242. Likewise, there may bean HP-EGR system (shown at FIG. 3) wherein HP-EGR is routed through anHP-EGR passage from upstream of turbine 176 to downstream of compressor174. The amount of HP-EGR provided to intake passage 146 may be variedby controller 12 via a dedicated HP-EGR valve. The HP-EGR system mayinclude an HP-EGR cooler (see FIG. 3) and the LP-EGR system may includeLP-EGR cooler 246 to reject heat from the EGR gases to engine coolant,for example.

Under some conditions, the EGR system may be used to regulate thetemperature of the air and fuel mixture within combustion chamber 30.Thus, it may be desirable to measure or estimate the EGR mass flow. EGRsensors may be arranged within EGR passages and may provide anindication of one or more of mass flow, pressure, temperature,concentration of O₂, and concentration of the exhaust gas. In someembodiments, one or more sensors may be positioned within LP-EGR passage240 to provide an indication of one or more of a pressure, temperature,and air-fuel ratio of exhaust gas recirculated through the LP-EGRpassage. Exhaust gas diverted through LP-EGR passage 240 may be dilutedwith fresh intake air at a mixing point located at the junction ofLP-EGR passage 240 and intake passage 142. Specifically, by adjustingLP-EGR valve 242 in coordination with a low pressure air-inductionsystem (LP AIS) throttle 230 (further elaborated at FIG. 3), a dilutionof the EGR flow may be adjusted.

A percent dilution of the LP-EGR flow may be inferred from the output ofa sensor 245 in the EGR gas stream. Specifically, sensor 245 may bepositioned downstream of LP-EGR valve 242, such that the LP-EGR dilutionmay be accurately determined. Sensor 245 may be, for example, an EGRdelta pressure over orifice, delta pressure over valve or hot wire orhot film anemometer flow meter. An oxygen sensor such as a UEGO sensor372 can also be used to measure EGR in the main intake duct 142 or 144.

Controller 12 is shown in FIG. 2 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; absolute manifold pressure signal (MAP) from sensor124, cylinder AFR from EGO sensor 128, and abnormal combustion from aknock sensor. Engine speed signal, RPM, may be generated by controller12 from signal PIP. Manifold pressure signal MAP from a manifoldpressure sensor may be used to provide an indication of vacuum, orpressure, in the intake manifold.

Storage medium read-only memory 110 can be programmed with computerreadable data representing instructions executable by processor 106 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed.

Now turning to FIG. 3, an example embodiment 300 of an engine system 10(such as the engine system of FIGS. 1-2) including a plurality ofcylinder banks and an exhaust gas recirculation system is illustrated.Embodiment 300 depicts a turbocharged engine system including amulti-cylinder internal combustion engine 10 and twin turbochargers 320and 330. As one non-limiting example, engine system 300 can be includedas part of a propulsion system for a passenger vehicle. Engine system300 can receive intake air via intake passage 340. Intake passage 340can include an air filter 356 and an EGR throttle valve 230. Enginesystem 300 may be a split-engine system wherein intake passage 340 isbranched downstream of EGR throttle valve 230 into first and secondparallel intake passages, each including a turbocharger compressor.Specifically, at least a portion of intake air is directed to compressor322 of turbocharger 320 via a first parallel intake passage 342 and atleast another portion of the intake air is directed to compressor 332 ofturbocharger 330 via a second parallel intake passage 344 of the intakepassage 340.

The first portion of the total intake air that is compressed bycompressor 322 may be supplied to intake manifold 360 via first parallelbranched intake passage 346. In this way, intake passages 342 and 346form a first parallel branch of the engine's air intake system.Similarly, a second portion of the total intake air can be compressedvia compressor 332 where it may be supplied to intake manifold 360 viasecond parallel branched intake passage 348. Thus, intake passages 344and 348 form a second parallel branch of the engine's air intake system.As shown in FIG. 3, intake air from intake passages 346 and 348 can berecombined via a common intake passage 349 before reaching intakemanifold 360, where the intake air may be provided to the engine.

A first EGR throttle valve 230 may be positioned in the engine intakeupstream of the first and second parallel intake passages 342 and 344,while a second air intake throttle valve 358 may be positioned in theengine intake downstream of the first and second parallel intakepassages 342 and 344, and downstream of the first and second parallelbranched intake passages 346 and 348, for example, in common intakepassage 349.

In some examples, intake manifold 360 may include an intake manifoldpressure sensor 382 for estimating a manifold pressure (MAP) and/or anintake manifold temperature sensor 383 for estimating a manifold airtemperature (MCT), each communicating with controller 12. Intake passage349 can include a charge air cooler (CAC) 354 and/or a throttle (such assecond throttle valve 358). The position of throttle valve 358 can beadjusted by the control system via a throttle actuator (not shown)communicatively coupled to controller 12. An anti-surge valve 352 may beprovided to selectively recirculate flow through the compressor stagesof turbochargers 320 and 330 via recirculation passage 350. As oneexample, anti-surge valve 352 can open to enable flow throughrecirculation passage 350 when the intake air pressure upstream of thecompressors attains a threshold value.

Air duct 349 may further include an intake gas oxygen sensor 372. In oneexample, the oxygen sensor is a UEGO sensor. The intake gas oxygensensor may be configured to provide an estimate regarding the oxygencontent of fresh air received in the intake manifold. In addition, whenEGR is flowing, a change in oxygen concentration at the sensor may beused to infer an EGR amount and used for accurate EGR flow control. Inthe depicted example, oxygen sensor 372 is positioned upstream ofthrottle 358 and downstream of charge air cooler 354. However, inalternate embodiments, the oxygen sensor may be positioned upstream ofthe CAC. A pressure sensor 374 may be positioned alongside the oxygensensor for estimating an intake pressure at which an output of theoxygen sensor is received. Since the output of the oxygen sensor isinfluenced by the intake pressure, a reference oxygen sensor output maybe learned at a reference intake pressure. In one example, the referenceintake pressure is a throttle inlet pressure (TIP) where pressure sensor374 is a TIP sensor. In alternate examples, the reference intakepressure is a manifold pressure (MAP) as sensed by MAP sensor 382.

Engine 10 may include a plurality of cylinders 30. In the depictedexample, engine 10 includes six cylinders arrange in a V-configuration.Specifically, the six cylinders are arranged on two banks 313 and 315,with each bank including three cylinders. In alternate examples, engine10 can include two or more cylinders such as 3, 4, 5, 8, 10 or morecylinders. These various cylinders can be equally divided and arrangedin alternate configurations, such as V, in-line, boxed, etc. Eachcylinder 30 may be configured with a fuel injector 66. In the depictedexample, fuel injector 66 is a direct in-cylinder injector. However, inother examples, fuel injector 66 can be configured as a port based fuelinjector.

Intake air supplied to each cylinder 30 (herein, also referred to ascombustion chamber 30) via common intake passage 349 may be used forfuel combustion and products of combustion may then be exhausted fromvia bank-specific parallel exhaust passages. In the depicted example, afirst bank 313 of cylinders of engine 10 can exhaust products ofcombustion via a first parallel exhaust passage 317 and a second bank315 of cylinders can exhaust products of combustion via a secondparallel exhaust passage 319. Each of the first and second parallelexhaust passages 317 and 319 may further include a turbocharger turbine.Specifically, products of combustion that are exhausted via exhaustpassage 317 can be directed through exhaust turbine 324 of turbocharger320, which in turn can provide mechanical work to compressor 322 viashaft 326 in order to provide compression to the intake air.Alternatively, some of the exhaust gases flowing through exhaust passage317 can bypass turbine 324 via turbine bypass passage 323 as controlledby wastegate 328. Similarly, products of combustion that are exhaustedvia exhaust passage 319 can be directed through exhaust turbine 334 ofturbocharger 330, which in turn can provide mechanical work tocompressor 332 via shaft 336 in order to provide compression to intakeair flowing through the second branch of the engine's intake system.Alternatively, some of the exhaust gas flowing through exhaust passage319 can bypass turbine 334 via turbine bypass passage 333 as controlledby wastegate 338.

In some examples, exhaust turbines 324 and 334 may be configured asvariable geometry turbines, wherein controller 12 may adjust theposition of the turbine impeller blades (or vanes) to vary the level ofenergy that is obtained from the exhaust gas flow and imparted to theirrespective compressor. Alternatively, exhaust turbines 324 and 334 maybe configured as variable nozzle turbines, wherein controller 12 mayadjust the position of the turbine nozzle to vary the level of energythat is obtained from the exhaust gas flow and imparted to theirrespective compressor. For example, the control system can be configuredto independently vary the vane or nozzle position of the exhaust gasturbines 324 and 334 via respective actuators.

Exhaust gases in first parallel exhaust passage 317 may be directed tothe atmosphere via branched parallel exhaust passage 370 while exhaustgases in second parallel exhaust passage 319 may be directed to theatmosphere via branched parallel exhaust passage 380. Exhaust passages370 and 380 may include one or more exhaust after-treatment devices,such as a catalyst, and one or more exhaust gas sensors.

Engine 10 may further include one or more exhaust gas recirculation(EGR) passages, or loops, for recirculating at least a portion ofexhaust gas from the exhaust manifold to the intake manifold. These mayinclude high-pressure EGR loops for proving high-pressure EGR (HP-EGR)and low-pressure EGR-loops for providing low-pressure EGR (LP-EGR). Inone example, HP-EGR may be provided in the absence of boost provided byturbochargers 320, 330, while LP-EGR may be provided in the presence ofturbocharger boost and/or when exhaust gas temperature is above athreshold. In still other examples, both HP-EGR and LP-EGR may beprovided simultaneously.

In the depicted example, engine 10 may include a low-pressure EGR loop202 for recirculating at least some exhaust gas from the first branchedparallel exhaust passage 370, downstream of the turbine 324, to thefirst parallel intake passage 342, upstream of the compressor 322. Insome embodiments, a second low-pressure EGR loop (not shown) may belikewise provided for recirculating at least some exhaust gas from thesecond branched parallel exhaust passage 380, downstream of the turbine334, to the second parallel intake passage 344, upstream of thecompressor 332. LP-EGR loop 202 may include LP-EGR valve 204 forcontrolling an EGR flow (i.e., an amount of exhaust gas recirculated)through the loops, as well as an EGR cooler 206 for lowering atemperature of exhaust gas flowing through the EGR loop beforerecirculation into the engine intake. Under certain conditions, the EGRcooler 206 may also be used to heat the exhaust gas flowing throughLP-EGR loop 202 before the exhaust gas enters the compressor to avoidwater droplets impinging on the compressors.

Engine 10 may further include a first high-pressure EGR loop 208 forrecirculating at least some exhaust gas from the first parallel exhaustpassage 317, upstream of the turbine 324, to the first branched parallelintake passage 346, downstream of the compressor 322. Likewise, theengine may include a second high-pressure EGR loop (not shown) forrecirculating at least some exhaust gas from the second parallel exhaustpassage 318, upstream of the turbine 334, to the second branchedparallel intake passage 348, downstream of the compressor 332. EGR flowthrough HP-EGR loops 208 may be controlled via HP-EGR valve 210. Assuch, HP-EGR may be injected downstream of the engine throttle 358 toimprove the flow capability under some operating conditions.Alternatively, the HP EGR loop(s) may include an EGR cooler (not shown).

A PCV port 302 may be configured to deliver crankcase ventilation gases(blow-by gases) to the engine intake manifold along second parallelintake passage 344. In some embodiments, flow of PCV air through PCVport 302 (e.g., PCV flow) may be controlled by a dedicated PCV portvalve. Likewise, a purge port 304 may be configured to deliver purgegases from a fuel system canister to the engine intake manifold alongpassage 344. In some embodiments, flow of purge air through purge port304 may be controlled by a dedicated purge port valve.

Humidity sensor 232 and pressure sensor 234 may be included in only oneof the parallel intake passages (herein, depicted in the first parallelintake air passage 342 but not in the second parallel intake passage344), downstream of EGR throttle valve 230. Specifically, the humiditysensor and the pressure sensor may be included in the intake passage notreceiving the PCV or purge air. Humidity sensor 232 may be configured toestimate a relative humidity of the intake air. In one embodiment,humidity sensor 232 is a UEGO sensor configured to estimate the relativehumidity of the intake air based on the output of the sensor at one ormore voltages. Since purge air and PCV air can confound the results ofthe humidity sensor, the purge port and PCV port are positioned in adistinct intake passage from the humidity sensor. Alternatively, theymay be positioned downstream of the humidity sensor. Pressure sensor 234may be configured to estimate a pressure of the intake air. In someembodiments, a temperature sensor may also be included in the sameparallel intake passage, downstream or upstream of the EGR throttlevalve 230.

Intake oxygen sensor 372 may be used for estimating an intake air oxygenconcentration and inferring an amount of EGR flow through the enginebased on a change in the intake oxygen concentration upon opening of theEGR valve 204. Specifically, a change in the output of the sensor uponopening the EGR valve is compared to a reference point where the sensoris operating with no EGR (the zero point). Based on the change (e.g.,decrease) in oxygen amount from the time of operating with no EGR, anEGR flow currently provided to the engine can be calculated. Forexample, upon applying a reference voltage (Vs) to the sensor, a pumpingcurrent (Ip) is output by the sensor. The change in oxygen concentrationmay be proportional to the change in pumping current (delta Ip) outputby the sensor in the presence of EGR relative to sensor output in theabsence of EGR (the zero point). Based on a deviation of the estimatedEGR flow from the expected (or target) EGR flow, further EGR control maybe performed.

The position of intake and exhaust valves of each cylinder 30 may beregulated via hydraulically actuated lifters coupled to valve pushrods,or via a cam profile switching mechanism in which cam lobes are used.Specifically, the intake valve cam actuation system 325 may include oneor more cams and may utilize variable cam timing or lift for intakeand/or exhaust valves. In alternative embodiments, the intake valves maybe controlled by electric valve actuation. Similarly, the exhaust valvesmay be controlled by cam actuation systems or electric valve actuation.

Engine system 300 may be controlled at least partially by a controlsystem 15 including controller 12 and by input from a vehicle operatorvia an input device (not shown). Control system 15 is shown receivinginformation from a plurality of sensors 16 (various examples of whichare described herein) and sending control signals to a plurality ofactuators 81. As one example, sensors 16 may include humidity sensor232, intake air pressure sensor 234, MAP sensor 382, MCT sensor 383, TIPsensor 374, and intake air oxygen sensor 372. In some examples, commonintake passage 349 may further include a throttle inlet temperaturesensor for estimating a throttle air temperature (TCT). In otherexamples, one or more of the EGR passages may include pressure,temperature, and hot-wire or hot-film anemometer flow sensors, fordetermining EGR flow characteristics. As another example, actuators 81may include fuel injector 66, HP-EGR valve 210, LP-EGR valve 204,throttle valves 358 and 230, and wastegates 328, 338. Other actuators,such as a variety of additional valves and throttles, may be coupled tovarious locations in engine system 300. Controller 12 may receive inputdata from the various sensors, process the input data, and trigger theactuators in response to the processed input data based on instructionor code programmed therein corresponding to one or more routines. Anexample control routine is described herein with regard to FIG. 4.

Now turning to FIG. 4, an example routine 400 is shown for operating thevehicle system of FIG. 1. Specifically, the method of FIG. 4 enables lowpressure EGR to be rapidly purged from the engine system of FIGS. 2-3. Acontroller may selectively operate a generator of the vehicle system tospin the engine unfueled during decreasing load conditions to rapidlyreplace intake EGR with fresh intake air. Alternatively, if the systembattery is capable of accepting charge, the controller may operate theengine in a generating mode with EGR disabled to replace intake EGR withfresh air while using the excess engine torque generated to charge thebattery. In both cases, the dilution tolerance of the engine system isimproved, likelihood of misfires is reduced, and peaks EGR ratesachievable during subsequent high load operations is increased.

At 402, engine operating conditions and vehicle operating parameters areestimated and/or measured. These include, for example, a brake pedalposition, accelerator pedal position, operator torque demand, batterystate of charge (SOC), engine temperature (Teng), ambient temperatureand humidity, barometric pressure (BP), etc. In one example, the hybridvehicle system is a power split hybrid vehicle system.

At 404, a vehicle mode of operation may be determined based on theestimated operating conditions. For example, based at least on theestimated driver torque demand and the battery state of charge, it maybe determined whether the vehicle is to be operated in an engine-onlymode (with the engine driving the vehicle wheels), an assist mode (withthe battery assisting the engine in driving the vehicle), or anelectric-only mode (with only the battery driving the vehicle). In oneexample, if the demanded torque can be provided by only the battery, thevehicle may be operated in the electric-only mode. In another example,if the demanded torque cannot be provided by the battery, the vehiclemay be operated in the engine mode, or in the assist mode. The vehiclemay accordingly be operated in the determined mode of operation.

At 406, it may be confirmed that the vehicle is in an engine-on mode.For example, it may be confirmed that the vehicle is in an engine-onlymode where the vehicle is being propelled with engine torque only.Alternatively, it may be confirmed that the vehicle is in an assist modeand that the vehicle is being propelled, at least in part, with enginetorque. If the engine-on mode is not confirmed, the routine may end.

At 408, fuel may be delivered to the engine to operate the engine andgenerate engine torque for propelling the vehicle. Herein, the engineoutput torque may correspond to a torque demanded for vehiclepropulsion. In one example, based on the engine operating conditions,such as the engine speed-load conditions, the engine may be operatingboosted with fuel delivered to the boosted engine via direct injection.In addition, the engine may be operating with EGR enabled or flowing.Herein, operating with EGR includes operating with low pressure EGR(LP-EGR) flowing. By flowing LP-EGR during engine operation, fueleconomy is improved via pumping work reduction, knock mitigation,combustion efficiency improvement, and enrichment reduction. Operatingthe engine with low pressure EGR includes operating the engine with anEGR valve coupled in an LP-EGR passage open.

In one example, the LP-EGR schedule may be a flat schedule whereinLP-EGR is delivered at a fixed rate relative to airflow. The LP-EGRincludes cooled exhaust residuals recirculated from an engine exhaustmanifold, downstream of an exhaust turbine, to an engine intakemanifold, upstream of an intake compressor.

As such, the amount of EGR (HP-EGR, LP-EGR, or a combination of the two)delivered to the intake may be based on an engine-speed load map storedin the memory of controller 12. In one example, the engine speed-loadmap may include at least two LP-EGR operating modes, including a fixedand a variable operating mode. The fixed mode range may comprise allengine speeds and loads over the normal range of engine operation. Thefixed mode may “taper out” LP EGR at very high engine speeds and loadsincluding full load to avoid conflicts with engine performance. Incomparison, the variable EGR mode may allow for variable EGR rates overthe normal range of engine operation.

At 410, it may be determined if decreasing engine load conditions arepresent. Specifically, it may be determined if there is a drop in enginetorque demand and decreasing engine torque demand conditions arepresent. For example, decreasing engine load/demand may be responsive toan operator pedal tip-out. As such, during the decreasing engineload/demand, airflow to the engine may be decreased and correspondingly,LP-EGR to the engine may also be decreased. However, due to a largetransport delay between the LP-EGR valve and the combustion chamber, EGRmay not be decreased as fast as required. Specifically, since the LP-EGRpassage takes off exhaust gas after the turbine and injects exhaust gasbefore the compressor, there is a large delay in purging of the EGR fromthe intake manifold. The delay is exacerbated by the presence of a largeboosted volume (e.g., six times the volume) over engine displacement insome engine configurations. The delay in purging leads to combustionstability risks. For example, the presence of more dilution thanrequired can increase the likelihood of misfires.

The adoption of the flat EGR schedule, wherein the EGR rate is keptconstant relative to airflow, helps in alleviating some of the issuesassociated with the delayed purging. However, the use of a flat scheduleresults in operating LP-EGR at some lower load points where no fueleconomy benefit is achieved. In fact, in some lower engine speed-loadpoints, operating LP-EGR results in a fuel penalty. In addition, thecompressor may be exposed to EGR at the low load conditions,necessitating corrosion and condensation countermeasures. As EGR flowsthrough the charge air cooler, additional condensation may arise thatmay also need to be addressed. At some low load conditions, a lowpressure air intake throttle may also need to be operated to drive theEGR flow. Furthermore, the lower load points limit the flat EGR scheduleat higher load points as the lower load points are the points where thecombustion system is the most dilution (EGR) limited. For example, theflat schedule limits the peak EGR rate achievable at higher enginespeed-load conditions. As such, this limits the fuel economy benefit ofLP-EGR.

FIG. 5 depicts an example flat EGR schedule at map 500. As showntherein, a flat schedule with EGR delivered at a fixed rate relative toairflow is applied in engine speed-load regions corresponding to zone502. Outside of zone 502, in zone 504, no LP-EGR is used. While thedelivery of cooled LP-EGR at a fixed rate provides significant fueleconomy benefits in the mid speed-load region (upper half of zone 502),the benefit may be limited. As such, it may be desirable to operate withmore LP-EGR in this region. However, due to the large EGR transportdelay, this may not be achievable. In addition, the flat schedule of map500 results in operating LP-EGR at the low engine speed-load conditions(lower half of zone 502) where a potential fuel penalty may be incurred.As such, it may be desirable to not operate with any EGR in this region.However, due to the delivery of EGR at a pre-compressor location, andthe resulting large EGR transport delay, this may not be achievable.

The inventors herein have recognized that EGR purging can be expeditedby operating a motor of the hybrid vehicle to replace the EGR in theintake manifold with fresh intake air. In particular, in response todecreasing engine torque demand or load while operating an engine withEGR flow, fuel to the engine may be disabled and the engine may be spununfueled via the motor until EGR is sufficiently purged from the intake.Alternatively, if a system battery is capable of accepting charge, EGRpurging can be expedited by disabling EGR and operating the engine in agenerating mode to replace intake EGR with fresh air, while using theexcess engine torque to charge the system battery.

For example, by expediting the purging of LP-EGR, an EGR schedule suchas the schedule of map 550 of FIG. 5 may be achieved. Specifically, byallowing for rapid purging of LP-EGR at low load conditions, it may bepossible to operate with no LP-EGR at low engine speed-load regions,such as shown at zone 554. By decreasing running of cooled LP-EGR atlower loads, and relying instead on hot internal EGR, fuel economy andengine performance is improved in this operating region. In addition, byallowing for rapid purging and resetting of EGR to substantially zeroLP-EGR conditions at decreasing engine loads, higher peak EGR rates canbe achieved during subsequent increasing engine loads. For example, itmay be possible to operate with higher LP-EGR rates at medium speed-loadregions, such as show at zone 552.

Returning to FIG. 4, in response to decreasing engine load and torquedemand, at 411, the battery state of charge (SOC) may be estimatedand/or measured and compared to a threshold charge. The routine furtherdetermines if the battery state of charge is higher than a thresholdcharge. The threshold charge may be defined to allow for a smalladditional engine-on time and subsequent charging to enable the EGRpurge.

If the battery state of charge is higher than the threshold charge, thenit may be determined that the battery is not capable of acceptingfurther charge. Accordingly, at 412, in response to the decreasingengine torque demand, engine fueling is disabled while a motor/generatorof the hybrid vehicle system is operated. As a result, while the engineis disabled, the vehicle is propelled using motor torque instead ofengine torque. At 414, to expedite purging of LP-EGR from the engineintake manifold, the routine includes spinning the engine unfueled viathe motor/generator. For example, the engine may be spun unfueled for anadditional 1-3 seconds via the generator. In addition, while spinningthe engine, each of an EGR valve in a LP-EGR passage and an intakethrottle in the intake passage may be fully opened. By opening the EGRvalve and the intake throttle fully during the spinning, the EGR systemas well as the air induction system may be purged of exhaust residualsand replenished with fresh intake air.

Spinning the engine unfueled via the motor includes operating thegenerator using electrical energy from the system battery to spin theengine at a selected engine speed. The engine may be spun unfueled at aselected engine speed that is based on the engine speed before the fuelinjectors are shut-off. For example, the controller may operator thegenerator to maintain the engine speed that the engine was spinning atimmediately before the fuel injectors were disabled. As another example,the generator may spin the engine unfueled at an engine speed that is afunction (e.g., fraction) of the engine speed that the engine wasspinning at immediately before the fuel injectors were disabled.Alternatively, the selected engine speed may be a speed that isefficient for both the engine and the transmission. As such, the purgetime required to completely purge the EGR will be a function of enginespeed and throttle position.

In an alternate example, the engine may be spun unfueled at a speedbased on the vehicle speed. For example, the engine speed may be set tobe a calibratable speed that is stored in the controller's memory in alook-up table accessed as a function of the vehicle speed. In yetanother example, the engine may be spun at a speed based on the vehiclespeed and a rotational speed (or rotational speed limit) of the rotatingcomponents of the planetary gear transmission. Motor/generator settingsmay be adjusted to enable the engine to be spun, via motor torque, atthe selected engine speed. In some embodiments, each of the generatorand the motor may be operated to spin the engine at the selected enginespeed. In other embodiments, only the generator may need to be operated.

In yet another example, the engine may be spun unfueled at an enginespeed corresponding to at least a cranking speed of the engine. Inaddition to expediting EGR purging, this allows the engine to be rapidlyrestarted in the event of a driver change-of-mind operation (such aswhere the operator tips-out and then tips-in soon after). For example,in response to an indication of an operator change of mind, thecontroller may start to fuel the engine and spin up the engine from thecranking speed so as to meet operator torque demand.

In still other examples, the engine may be spun unfueled at an enginespeed that allows the EGR to be purged as fast as possible. Herein, theengine speed may be selected based on the intake EGR level at a time ofthe decreasing engine torque demand (e.g., at a time of operator pedaltip-out). For example, the engine speed may be transiently raised to amaximum allowable engine speed that does not affect torque output butthat allows EGR to be purged as fast as possible. In yet anotherexample, the engine may be spun unfueled at an engine speed that allowsthe EGR to be purged at a slower rate. For example, the operator pedaltip-out and decreasing engine load/torque demand may occur during adownhill vehicle travel. The operator may indicate a long downhilltravel segment by pressing a button on the vehicle dashboard or via aninteractive display on a center console of the vehicle. By indicating along downhill travel, the operator may indicate that the engine may beshut down for a longer duration. Accordingly, during the downhilltravel, the engine may be spun unfueled via the generator so that EGRpurging can be completed by the time the downhill travel is completed.

In further examples, instead of spinning the engine continuously untilEGR is purged, the engine may be spun unfueled via the generatorintermittently. For example, during a downhill travel, the engine may bepulsed unfueled via the generator to purge the EGR.

At 416, it may be determined if the EGR has been sufficiently purgedfrom the engine intake manifold. For example, it may be determined ifEGR (flow, amount, concentration, level, etc.) in the intake is lowerthan a threshold. In one example, an intake oxygen sensor, such assensor 372 of FIG. 3, may be used to estimate the concentration of EGRin the intake. Therein, a drop in intake oxygen concentration may beused to infer an increase in EGR dilution delivery. In one example, thethreshold is based on EGR tolerance of the engine at low engine loadconditions. For example, as the EGR tolerance increases, the thresholdmay be increased.

If the LP-EGR is not lower than the threshold, then the controller maycontinue to spin the engine unfueled via the motor/generator until EGRis sufficiently purged. If EGR is lower than the threshold, then at 420,the routine includes spinning the engine to rest. For example, theengine may be spun to rest via the motor and thereafter the engine maybe maintained shutdown until engine restart conditions are met. In themeantime, the vehicle may continue to be propelled using motor torque.As such, this allows the LP-EGR rate to be reset (for example, to zero)such that when the engine is restarted, a known, higher LP-EGR rate canbe realized to improve engine efficiency in the key medium load regionof the speed-load map.

At 430, it may be determined if engine restart conditions are met. Forexample, the engine may be restarted in response to one or more of thebattery state of charge being lower than a threshold level of charge, arequest for air conditioning being received, operator torque demandbeing higher than a threshold amount, etc. If engine restart conditionsare not met, the engine may be maintained shutdown and the vehicle maycontinue to be propelled via the motor. Else, at 432, in response torestart conditions being met, the engine may be restarted and enginefueling may be resumed. Herein, upon restarting the engine, EGR may beenabled and higher EGR flow rates may be achieved since the engine wasalready purged.

Returning to 411, if the battery SOC is lower than the threshold charge,then it may be determined that the battery is capable of acceptingfurther charge. Consequently, EGR purging may be enabled by transientlyoperating the vehicle in a generating mode. Specifically, at 422, theroutine includes disabling EGR in response to the decreasing engine loadconditions. Disabling EGR includes closing an EGR valve coupled in anLP-EGR passage to disable further recirculation of exhaust residualsfrom the exhaust manifold, downstream of the turbine, to the intakemanifold, upstream of the compressor.

At 424, the routine includes operating the engine with EGR disabled andwith engine output torque higher than demanded torque. That is, untilEGR in the engine intake is sufficiently purged, the engine may continueto be spun fueled with engine output torque generated in excess oftorque demanded for vehicle propulsion. By operating the engine fueledwith the EGR valve closed, fresh intake air drawn into the air inductionsystem may replace the intake EGR, expediting EGR purging. In oneexample, the engine is operated fueled with the EGR valve closed for 1-3seconds.

Also at 424, the routine includes charging the system battery with theexcess engine output torque. That is, the system battery is charged withengine output torque, in excess of demanded torque, generated duringengine operation with EGR disabled. Charging the battery may includeoperating the generator using the excess engine output torque, thegenerator coupled to the battery. In one example, the engine is operatedin the generating mode for 1-3 seconds.

Operating the engine with engine output torque higher than demandedincludes operating the engine at an engine speed that is based on one ormore of a state of charge of the system battery and an EGR level of theengine intake at a time of the decreasing engine torque demandcondition. For example, the engine speed may be based on a chargeaccepting ability of the battery. Thus, as a difference between thebattery SOC and the threshold charge increases (and thereby the chargeaccepting capability of the battery increases), the engine speed atwhich the engine is operated may be increased. The engine speed may alsobe based on the EGR level in the intake at the time of an operator pedaltip-out. For example, as the EGR level at tip-out increases, morepurging may be required, and consequently the engine speed may beincreased. In still further examples, the engine speed may be furtheradjusted based on vehicle speed.

At 426, as at 416, it may be determined if the LP-EGR has beensufficiently purged from the engine intake manifold. For example, it maybe determined if EGR (flow, amount, concentration, level, etc.) in theintake is lower than the threshold. The threshold may be based on theEGR tolerance of the engine at low engine load conditions.

If the LP-EGR is not lower than the threshold, then the controller maycontinue to operate the engine fueled, with EGR disabled and with excessengine torque generated, and with the excess torque stored as charge ina system battery, until the EGR is sufficiently purged. At 428, when EGRin the intake is less than the threshold, the routine includesdeactivating fuel to the engine and spinning the down the engine torest. Thereafter the engine may be maintained shutdown until enginerestart conditions are met. In the meantime, the vehicle may bepropelled using motor torque from the motor/generator. As such, thisallows the LP-EGR rate to be reset (for example, to zero) such that whenthe engine is restarted, a known, higher LP-EGR rate can be realized toimprove engine efficiency in the key medium load region of thespeed-load map.

From 428, the routine proceeds to 430 to determine if engine restartconditions are met, and restart the engine at 432 if conditions are met.Upon restarting the engine, EGR may be enabled and higher EGR flow ratesmay be achieved since the engine was already purged.

In this way, at 422-428, during a tip-out from operating an engine withEGR flowing, while a battery state of charge is lower than a thresholdcharge, a battery may be charged by operating the engine with EGRdisabled until an engine intake EGR level is lower than a threshold, theengine operated to generate more torque than demanded. Herein, operatingthe engine with EGR flowing includes operating the engine with an EGRvalve, coupled in a low pressure EGR passage, open (e.g., fully open)and operating the engine with EGR disabled includes operating with theEGR valve closed (e.g., fully closed). Once the engine intake EGR levelis lower than the threshold, engine operation and battery charging maybe discontinued.

It will be appreciated that while the routine of FIG. 4 shows selectingbetween purging LP-EGR in a hybrid vehicle system by spinning an engineunfueled via a generator or operating an engine fueled with EGR disabledand with a battery being charged based on a charge accepting capability(or SOC) of a system battery, in alternate examples, the controller maybe configured to select based on the LP-EGR level in the engine intakeduring the decreasing engine load/torque demand conditions. For example,if the LP-EGR level at a time of operator pedal tip-out is higher, thecontroller may select purging by spinning the engine unfueled via thegenerator. Else, if the LP-EGR level at the time of operator pedaltip-out is lower, the controller may select purging by operating theengine fueled in a generating mode with EGR disabled. Further still, insome examples, the controller may be configured to select spinning theengine unfueled via the generator as the default purging option. Thecontroller may then selectively override the default purging option withthe generating mode purging option based on operator input.

It will be appreciated that while the routine of FIG. 4 depicts EGRpurging in response to decreasing engine load/torque demand (such as dueto an operator tip-out), in alternate examples, the EGR purging may beinitiated in anticipation of an engine shutdown. For example, based onvehicle operating conditions, the vehicle controller may determine animminent an engine shutdown and may start the EGR purging before theanticipated engine shutdown occurs. Herein, the EGR purging may beperformed independent of the operator input, for example, independent ofthe operator pedal tip-out event or driver demand. Rather, the EGRpurging may be performed based on vehicle operating conditions (e.g.,vehicle speed, ambient humidity, etc.) which may determine a frequencyof engine shutdown and restart. By initiating the EGR purging inanticipation of an engine shutdown, additional purge delays are reduced.

In this way, LP-EGR can be rapidly purged and EGR levels can be resetduring decreasing engine torque demand conditions. The rapid purgingreduces combustion stability risks associated with lingering EGR at lowload conditions. In addition, the resetting of EGR levels allows higherEGR rates to be realized during increasing engine torque demand tomedium load conditions.

In one example, a hybrid vehicle system comprises an engine including anintake and an exhaust; an intake throttle; an electric motor/generatorcoupled to a battery; and vehicle wheels propelled using torque from oneor more of the engine and the motor. The hybrid vehicle system furtherincludes a direct fuel injector coupled to an engine cylinder; aturbocharger including an intake compressor driven by an exhaustturbine; and an EGR passage for flowing EGR from the exhaust, downstreamof the turbine, to the intake, upstream of the compressor, via an EGRvalve. The vehicle system may include a controller with computerreadable instructions for, during a tip-out from operating the enginewith EGR flowing, disabling the fuel injector; fully opening each of theEGR valve and the intake throttle; and using torque from the motor tomeet an operator torque demand and spin the engine unfueled, the enginespinning continued for a duration until EGR in the engine is lower thana threshold. Herein, spinning until LP-EGR in the engine is lower than athreshold includes spinning until an amount of LP-EGR in an intakemanifold of the engine is lower than the threshold, the threshold basedon the torque demand. In one example, spinning until LP-EGR in theengine is lower than a threshold includes spinning until LP-EGR flow isat zero flow. Spinning the engine includes spinning the engine at anengine speed at or above an engine cranking speed. The controller mayinclude further instructions for, after the duration, spinning theengine to rest and maintaining the engine shutdown while continuing touse motor torque to meet the torque demand.

In another example, the controller of the above described hybrid vehiclesystem includes computer readable instructions for, in response to anoperator pedal tip-out while operating the engine with EGR flowing,estimating a battery state of charge; and if the estimated battery stateof charge is lower than a threshold charge, closing the EGR valve;operating the engine with the EGR valve closed for a duration until EGRin the engine is lower than a threshold, the engine operated to generatemore torque than demanded; and charging the battery with excess enginetorque generated while operating the engine with the EGR valve closed.Herein, operating the engine with the EGR valve closed may includespinning the engine at an engine speed based on one or more of thebattery state of charge at the tip-out and an intake EGR level at thetip-out. Further, operating the engine with the EGR valve closed untilEGR in the engine is lower than a threshold may include operating theengine until EGR flow is at zero flow. The controller may includefurther instructions for, if the estimated battery state of charge ishigher than the threshold charge, disabling the fuel injector; fullyopening the EGR valve and the intake throttle; and using torque from thegenerator to meet operator torque demand and spin the engine unfueleduntil EGR in the engine is lower than the threshold.

Example EGR purging operations are now shown with reference to theexample of FIG. 6. Specifically, map 600 depicts engine speed at plot602, motor torque at plot 604, LP-EGR at plot 606, fuel injection atplot 608, a battery state of charge (SOC) at plot 610, and a position ofan LP-EGR valve at plot 614.

Prior to t1, the hybrid vehicle may be operating with a larger portionof wheel torque being provided by the engine and a smaller portion ofthe wheel torque being provided by the motor. Accordingly, the enginemay be spinning fueled (plot 608) with an engine speed corresponding tooperation in a medium to higher load region (plot 602) with only someassist from the motor (plot 604). While operating in the medium tohigher load region, LP-EGR may be flowing (plot 606), for example, witha flat schedule where EGR is provided at a fixed rate relative toairflow. Specifically, an LP-EGR valve may be open (plot 614). In thedepicted example, the LP-EGR valve is shown as an on-off valve that canbe shifted between a fully open and a fully closed position. However, inother examples, an opening of the EGR valve may be variably adjustedbased on the LP-EGR demand. During the engine operation prior to t1, thebattery state of charge may be higher than a threshold 612 and thebattery may not be capable of accepting further charge (plot 610).

At t1, an operator pedal tip-out may occur resulting in a decrease inengine load to low load conditions. In response to the decreasing enginetorque demand, EGR to the engine may be decreased. As such, if an LP-EGRvalve were adjusted (e.g., closed) to decrease the EGR, due to thepre-compressor location of EGR delivery, there may be a large transportdelay and EGR may not decrease as fast as desired. For example, EGR maydecrease as per the profile of dotted segment 609 a. This would resultin the presence of excess dilution in the engine intake manifold at lowload conditions, increasing the propensity for misfires and combustionstability issues.

To improve the purging of LP-EGR at the low load conditions and enableminimal (e.g., zero flow) EGR to be provided to the engine at the lowload conditions, EGR may be rapidly purged using assistance from asystem generator. Herein, purging with assistance from the generator maybe requested due to the battery state of charge being higher thanthreshold 612. Specifically, at t1, fuel injection to the engine isdisabled, causing a drop in engine speed. In addition, motor/generatoroutput is increased so as to provide sufficient motor torque to propelthe vehicle and meet the operator torque demand, while also providingsufficient motor torque to spin the engine unfueled. As such, if themotor/generator were not operated, the engine may spin down to rest, asper the profile of dotted segment 603. While spinning the engineunfueled via the motor/generator, the LP-EGR valve may be kept fullyopen. In addition, an intake throttle (not shown) may be fully opened.This allows EGR in the air induction system to be replaced with freshintake air rapidly.

Spinning the engine unfueled includes spinning the engine at an enginespeed 601. Engine speed 601 may be a cranking engine speed.Alternatively, engine speed 601 may correspond to engine speed beforefuel injector deactivation, or a function thereof. Further still, enginespeed 601 may correspond to an engine speed that is most efficient forthe engine and the transmission. As such, the engine may be spun via themotor for a duration between t1 and t2 until the LP-EGR is sufficientlypurged. For example, the engine may be spun at engine speed 601 untilLP-EGR is at or below a minimum EGR level 611. In an alternate example,EGR level 611 may include no EGR flow such that no LP-EGR is provided atlower engine load conditions.

At t2, once the EGR is sufficiently purged, the engine is allowed tostop. In addition, the LP-EGR valve is closed. Thereafter, the engine ismaintained shutdown until restart conditions are met (at t3). In themeantime, between t2 and t3, motor operation may be adjusted so thatenough motor torque is produced to propel the vehicle. Between t1 andt3, when the engine is not running and motor torque is being used topropel the vehicle and/or spin the engine unfueled, the battery SOC mayfall. For example, at t3, the battery SOC may drop below threshold 612.

At t3, in response to engine restart conditions (such as due to a risein operator torque demand), fuel injection to the engine may bereinitiated and engine torque may be increased to propel the vehicle. Atthe same time, the motor torque may be reduced since the vehicle ispropelled largely with engine torque. While the depicted example showsmotor torque being reduced to a lower level, in alternate example, useof motor torque may be completely discontinued. Also at t3, the LP-EGRvalve is opened to re-enable EGR during engine operation. Further, sinceLP-EGR was reset at t2, during the restart to higher loads at t3, higherLP-EGR peak rates may be delivered.

Vehicle operation with the engine operating and EGR being delivered maycontinue until t4. As such, sufficient time may elapse between t3 and t4(depicted by dotted lines). At t4, the hybrid vehicle may be operatingin an engine-only mode with wheel torque demand being met by the engine.The engine may be spinning fueled at an engine speed corresponding tooperation in a medium to higher load region, with LP-EGR flowing (andthe LP-EGR valve open). For example, LP-EGR may be provided as per aflat schedule with a fixed rate of EGR relative to airflow. During theengine operation at t4, the battery state of charge may be lower thanthreshold 612 and the battery may be capable of accepting furthercharge.

At t5, as at t1, an operator pedal tip-out may occur resulting in adecrease in engine load to low load conditions. In response to thedecreasing engine load, EGR to the engine may be decreased.Specifically, the LP-EGR valve is closed to decrease the EGR. However,even with EGR valve closing, due to the pre-compressor location of EGRdelivery, there may be a large transport delay and EGR may not decreaseas fast as desired. For example, EGR may decrease as per the profile ofdotted segment 609 b. This would result in the presence of excessdilution in the engine intake manifold at low load conditions,increasing the propensity for misfires and combustion stability issues.

To improve the purging of LP-EGR at the low load conditions and enableminimal (e.g., zero flow) EGR to be provided to the engine at the lowload conditions, EGR may be rapidly purged by operating the hybridvehicle system in a generating mode. Herein, purging by operating theengine in a generating mode may be requested due to the battery state ofcharge being lower than threshold 612 and the battery being able toaccept charge. Specifically, at t5, engine fueling and operation ismaintained but with EGR disabled. Fuel injection to the engine isadjusted to generate engine torque in excess of what is required topropel the vehicle and meet operator torque demand. In doing so, theengine is operated at a higher speed 605 than would have otherwise beenrequired (as shown by dashed segment 607) to propel the vehicle. Byoperating the engine at a higher speed with the EGR valve closed, EGR inthe air induction system can be rapidly replaced with fresh intake air,allowing for faster EGR purging. The excess torque generated by theengine is then used to charge the battery. Consequently, the batterystate of charge may start to rise after t5.

Operating the engine to generate excess torque includes spinning theengine at an engine speed 605. Engine speed 605 may be based on theLP-EGR level at the time of tip-out (at t5) as well as the battery SOCat the time of tip-out. As the battery state of charge decreases, ahigher engine speed 605 (relative to engine speed 607 that would beotherwise required to propel the vehicle) can be applied and higherlevels of excess torque can be generated to purge the EGR since thebattery is capable of accepting larger amounts of charge. Likewise, asthe LP-EGR level increases, and more purging is required, engine speed605 can be raised further relative to engine speed 607.

The engine is operated with EGR disabled and excess engine torquegenerated for a duration between t5 and t6 until the LP-EGR issufficiently purged. For example, the engine may continue be operated atengine speed 605 until LP-EGR is at or below minimum EGR level 611. Inan alternate example, EGR level 611 may include no EGR flow such that noLP-EGR is provided at lower engine load conditions. Also between t5 andt6, the battery state of charge may continue to increase until it ishigher than threshold 612 by t6.

At t6, once the EGR is sufficiently purged, the engine is allowed tostop. Specifically, engine fueling is disabled and the engine is allowedto spin to rest. Thereafter, the engine is maintained shutdown untilrestart conditions are met (at t7). In the meantime, between t6 and t7,a motor of the vehicle system is operated to generate sufficient motortorque to propel the vehicle.

At t7, in response to engine restart conditions (such as due to a risein operator torque demand), fuel injection to the engine may bereinitiated and engine torque may be increased to propel the vehicle. Atthe same time, the motor torque may be reduced (e.g., discontinued) suchthat the vehicle is propelled with engine torque. Further, since LP-EGRwas reset at t6, during the restart to higher loads at t7, higher LP-EGRpeak rates may be delivered.

In this way, during a first engine shutdown from operating with EGR, acontroller may disable fuel injection, and spin the engine unfueled viaa motor until the EGR is below a threshold. In comparison, during asecond engine shutdown from operating with EGR, the controller maydisable EGR, and spin the engine fueled until the EGR is below thethreshold while charging a battery with excess engine torque. Herein,during the first engine shutdown, while the engine is spinning unfueled,motor torque is used to propel the vehicle and spin the engine, whileduring the second engine shutdown, while the engine is spinning fueled,engine torque is used to propel the vehicle and charge the battery.Further, during the first engine shutdown, a state of charge of thebattery is above a threshold charge while during the second engineshutdown, the state of the charge of the battery is below the thresholdcharge. During the first engine shutdown, the engine is spun unfueled atan engine speed based on an engine speed before disabling fuelinjection, while during the second engine shutdown, the engine is spunfueled at an engine speed based on EGR level before disabling EGR.

In this way, during selected decreasing engine load/torque demandconditions, motor torque of a hybrid vehicle system can beadvantageously used to purge EGR in a low engine load region and improveEGR delivery in medium load regions. By controlling the motor speed toselectively spin the engine after fuel has been shut off, EGR trapped inthe boosted volume of the engine can be rapidly purged. During otherdecreasing engine load conditions, the charge accepting ability of ahybrid vehicle system battery can be advantageously used to purge EGR ina low engine load region. By enabling excess engine torque to be storedin the battery, the engine can be operated at higher engine speeds (andwith higher engine outputs) while EGR is disabled, allowing for EGR inthe air induction system to be rapidly replaced with fresh air. Byexpediting purging of the EGR, LP-EGR rates can be reduced faster at lowload conditions. For example, LP-EGR rates can be reset at the low loadconditions. This reduces the propensity for misfires and combustioninstability due to the presence of excess dilution at low loadconditions. The expedited purging further allows higher LP-EGR rates tobe realized when the engine is restarted. As such, this allows theengine to be used in its highest efficiency operating region. Inparticular, engine efficiency can be substantially improved in themedium load region. The interaction between the boosted engine and thepower-split hybrid application also allows for the fuel economypotential of LP-EGR to be improved and reduces compromises to EGR rates,such as the running of LP-EGR at low load conditions and the lower peakEGR rates achievable at medium to high load conditions when operatingwith a flat EGR schedule. Overall, vehicle performance and engine fueleconomy is improved.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory. The specific routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various actions, operations,and/or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedactions, operations and/or functions may be repeatedly performeddepending on the particular strategy being used. Further, the describedactions, operations and/or functions may graphically represent code tobe programmed into non-transitory memory of the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method for a hybrid vehicle system, comprising: in response todecreasing engine torque demand while operating an engine with EGR,disabling EGR; and until EGR in an engine intake is less than athreshold, operating an engine with EGR disabled and with engine outputtorque higher than demanded torque; and charging a system battery. 2.The method of claim 1, wherein operating the engine with EGR includesoperating the engine with low pressure EGR (LP-EGR) provided at a fixedrate relative to airflow, the LP-EGR including cooled exhaust residualsrecirculated from an exhaust manifold, downstream of an exhaust turbine,to the engine intake, upstream of an intake compressor via an EGRpassage.
 3. The method of claim 2, wherein operating the engine with EGRincludes operating the engine with an EGR valve in the EGR passage open.4. The method of claim 3, wherein disabling EGR includes closing an EGRvalve.
 5. The method of claim 1, wherein the threshold is based on anEGR tolerance of the engine at low engine load conditions.
 6. The methodof claim 1, wherein the decreasing engine torque demand is in responseto an operator pedal tip-out.
 7. The method of claim 1, whereinoperating the engine with engine output torque higher than demandedincludes operating the engine at an engine speed based on a state ofcharge of the battery and an EGR level at the decreasing engine torquedemand.
 8. The method of claim 1, wherein charging a system batteryincludes charging the battery with the engine output torque in excess ofdemanded torque generated during engine operation with EGR disabled. 9.The method of claim 8, wherein charging the system battery includesoperating a generator using the excess engine output torque, thegenerator coupled to the battery.
 10. The method of claim 1, furthercomprising, when EGR in the intake is less than the threshold,deactivating fuel to the engine and spinning down the engine to rest.11. The method of claim 1, wherein operating the engine with EGRincludes operating the engine boosted, with low pressure EGR flowing,and with fuel delivered to the boosted engine via direct injection. 12.The method of claim 1, wherein the hybrid vehicle system is a powersplit hybrid vehicle system.
 13. A method for a hybrid vehicle system,comprising: during a tip-out from operating an engine with EGR flowing,while a battery state of charge is lower than a threshold charge,charging the battery by operating the engine with EGR disabled until anengine intake EGR level is lower than a threshold, the engine operatedto generate more torque than demanded.
 14. The method of claim 13,wherein operating the engine with EGR flowing includes operating theengine with an EGR valve coupled in a low pressure EGR passage open, andwherein operating the engine with EGR disabled includes operating theengine with the EGR valve closed.
 15. The method of claim 14, whereinthe threshold charge is based on an EGR level of the intake at thetip-out, the threshold lowered as the EGR level increases.
 16. Themethod of claim 15, further comprising, after the engine intake EGRlevel is lower than the threshold, discontinuing engine operation anddiscontinuing battery charging.
 17. A hybrid vehicle system, comprising:an engine including an intake and an exhaust; an intake throttle; anelectric motor/generator coupled to a battery; vehicle wheels propelledusing torque from one or more of the engine and the motor; a direct fuelinjector coupled to an engine cylinder; a turbocharger including anintake compressor driven by an exhaust turbine; an EGR passage forflowing EGR from the exhaust, downstream of the turbine, to the intake,upstream of the compressor, via an EGR valve; and a controller withcomputer readable instructions for: in response to an operator pedaltip-out while operating the engine with EGR flowing, estimating abattery state of charge; and if the estimated battery state of charge islower than a threshold charge, closing the EGR valve; operating theengine with the EGR valve closed for a duration until EGR in the engineis lower than a threshold, the engine operated to generate more torquethan demanded; and charging the battery with excess engine torquegenerated while operating the engine with the EGR valve closed.
 18. Thesystem of claim 17, wherein operating the engine with the EGR valveclosed includes spinning the engine at an engine speed based on one ormore of the battery state of charge at the tip-out and an intake EGRlevel at the tip-out.
 19. The system of claim 18, wherein operating theengine with the EGR valve closed until EGR in the engine is lower than athreshold includes operating the engine until EGR flow is at zero flow.20. The system of claim 19, wherein the controller includes furtherinstructions for: if the estimated battery state of charge is higherthan the threshold charge, disabling the fuel injector; fully openingthe EGR valve and the intake throttle; and using torque from thegenerator to meet operator torque demand and spin the engine unfueleduntil EGR in the engine is lower than the threshold.